The present invention relates to a novel process for producing high purity sulfuric acid for use in the semiconductor industry.
Pure sulfuric acid can be produced on an industrial scale by passing SO3 into dilute sulfuric acid, by combining SO3 and pure water or by distillation of sulfuric acid at atmospheric or reduced pressure.
The quality of the sulfuric acid produced is affected not only by the design of the plants and the quality of the raw materials used, but also by the type and quality of the materials of construction used in the plant. These have an appreciable bearing on the level of undesirable metal ions, but also on the level of particles.
It is known to produce relatively high purity concentrated sulfuric acid by having SO3 evaporated or expelled from oleum in a distillation flask and then introduced into dilute pure sulfuric acid. In particular embodiments, the evaporation is occasionally carried out in a falling film evaporator. Generally, the equipment used for producing pure sulfuric acid is made of glass or of enamel-lined steel. Depending on the quality used, these materials may leach ionogenic and/or particulate impurities.
Existing processes have the disadvantage that, in the event of nonuniform evaporation, the gas stream may entrain drops of liquid in the form of a fine mist and any impurities present therein into the end product. This happens in particular on conducting the evaporation in falling film evaporators customarily used on an industrial scale, but also on using distillation flasks.
Another disadvantage is the SO2 still present in the sulfuric acid after purification.
It is an object of the present invention to provide an improved, economical way of producing on an industrial scale for use in the semiconductor industry a high purity sulfuric acid that is ideally free of metal ions and SO2, but ideally also free of particles in particular.
This object is achieved by a continuously operable process for producing high purity sulfuric acid for the semiconductor industry, which is characterized in that
a) hydrogen peroxide solution having a concentration of 1-70% is added to 24-70% technical grade oleum in a sufficient amount to lower the SO2 concentration to below 10 ppm,
b) the SO3 in the oleum is evaporated at 90-130xc2x0 C. in a falling film evaporator,
c) sulfuric acid and nitrosylsulfuric acid traces are removed from the SO3 gas stream escaping from the evaporator by means of a demister, for example in the form of a candle filter,
d) the high purity SO3 is enriched with inert gas, and
e) the SO3 is absorbed in sulfuric acid of a concentration of 90-99% with cooling.
High purity deionized water is added to adjust the concentration of the high purity sulfuric acid to a desired concentration, the concentration adjustment being closed loop controlled by conductivity measurement.